The present invention relates to a tactile sensor that detects viscoelastic characteristics of viscoelastic subjects.
For endscopes, importance has recently been set to a function of operating on a subject under observation rather than a function as an instrument for observing the interior of a living body. Endscope-based operations, such as are exemplified by enucleation of a gallbladder, and endscope-based diagnosis are expected to spread increasingly in the future.
In perforing appropriately more complicated, elaborate operations or diagnosis/treatment within a body cavity, not only visual information but also tactile information becomes more important.
Living bodies are viscoelastic mediums. The tactile sense (the sense of touch) described herein is defined as perception of a reactive force from a viscoelastic medium. A sensor used to detect the viscoelastic characteristics of a subject is referred to as a tactile sensor.
Using complex elastic modulus G*, the viscoelastic characteristics are generally represented by
G*=G'+jG" (1)
The real part G' in the complex elastic modulus G* corresponds to the elasticity and the imaginary part G" corresponds to the viscosity.
Using shear modulus .mu., the viscoelastic characteristics can also be represented by EQU .mu.*=.mu..sub.1 +j.omega..mu..sub.2 (2)
In living tissues, muscles and intertissue fluids are intermingled to exhibit the viscoelasticity. In morbid regions such as tumors, indurations, etc., both the real part G' and the imaginary part G" in the complex elastic modulus G* show different values from those in normal regions.
In order to measure the complex elastic modulus G*, it is required to measure the time-varying response of a subject. One measurement method involves giving vibrations to living tissues, measuring the response from the tissues, and determining the complex elastic modulus.
The qualitative detection of the viscoelasticity in that manner through the use of a tactile sensor allows for more accurate diagnosis of morbid regions.
In view of such a need, a method is described in "Physics of Vibrations in Living Tissues" by H. E. Gierke, et al., J. Applied Physiology, 4. 886/900 (1952), which involves exciting a vibrator put on human skin, calculating a mechanical impedance from the response, and solving a mechanical impedance-related theoretical formula derived in advance to obtain the elastic coefficient .mu..sub.1 and the viscosity coefficient .mu..sub.2 of the skin.
In addition, as a device for measuring the viscoelastic characteristics of a living body, a tactile sensor signal processing device has been disclosed in Jpn. Pat. Appln. KOKAI publication No. 9-96600 by way example, which makes use of changes in resonant resistance and resonant frequency of a piezoelectric vibrator to separate the viscoelastic characteristics of the living body into elasticity and viscosity.
As shown in FIG. 19, this type of tactile sensor signal processing device comprises a tactile sensor 101 for detecting the viscoelastic characteristics of a subject through changes in impedance characteristics of a piezoelectric vibrator, a resonant resistance change detector 102 for detecting a change in resonance resistance of the impedance characteristics, a resonant frequency change detector 103 for detecting a change in resonant frequency, and a signal processing unit 105 for calculating the real part and the imaginary part of the complex elastic modulus indicating the viscoelastic characteristics of the subject on the basis of the results of detection by the detectors 102 and 103.
In such an arrangement, if an oscillator circuit is used which uses equivalent-circuit constants of the piezoelectric vibrator as its circuit elements, then its output signal reflects the impedance characteristics of the vibrator.
When the oscillator circuit is in the no-load state, its output signal is outputted at close to the resonant frequency f.sub.r of the piezoelectric vibrator and its amplitude depends on the resonant resistance Z.sub.r of the vibrator.
When a viscoelastic medium is attached to the oscillator, the resonant frequency becomes f.sub.r ' and the resonant resistance changes to Z.sub.r '.
That is, the output signal of the oscillator reflects changes in impedance characteristics of the piezoelectric vibrator.
Thus, a change in resonant frequency and a change in resonant resistance can be detected by branching the oscillator output and converting the frequency component into a voltage signal for processing in the signal processing unit 105.
The resonance resistances Z.sub.r ' and Z.sub.r " detected by the resonant resistance change detector 102 and the resonant frequencies f.sub.r and f.sub.r ' detected by the resonant frequency change detector 103 are entered into the signal processing unit 105 and subjected to computational processing in accordance with a procedure incorporated in advance in the signal processing unit, so that the real part G' and the imaginary part G" of the complex elastic modulus G* are computed.
For example, if the equivalent-circuit constants (C.sub.1, L.sub.1, equipment constants K.sub.R, K.sub.L), the resonant resistance and the resonant frequency are known already, it is not until both a change in resonant frequency and a change in resonant resistance are measured that the real part G' and the imaginary part G" of the complex elastic modulus G* can be computed. The viscoelasticity can be detected, as indicated by ##EQU1##
In determining the mechanical impedance characteristics of a subject having viscoelasticity like a living body employing vibrations, it is required to keep the vibrator's state of contact with the subject unchanged or to reduce the measurement time because heartbeats cause movement of the living body.
In the conventional method by H. E. Gierke et al, the vibrator is excited with sinusoidal waves and the above-described .mu..sub.1 and .mu..sub.2 are computed by solving equations obtained at a limited number of typical frequencies.
Thus, this method involves a plurality of measurements for each measurement point and is subject to the constraint that the physical state of the subject to be measured must be kept stable during measurement and cannot therefore be said to be suitable for measurement in living bodies.
For the piezoelectric vibration phenomenon as in the piezoelectric vibrator, the solution must be found from an equation of motion, which is established from a piezoelectric equation so that electrical conditions are satisfied, under mechanical terminal conditions, and further the impedance and equivalent-circuit elements seen by electrical terminals must be determined from the electrical conditions.
The previously described tactile sensor signal processing device disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-96600, which computes the complex elastic modulus using the equipment constants K.sub.R and K.sub.L for converting the electrical impedance of a viscoelastic medium to the mechanical impedance, is difficult to use to compute the viscoelasticity accurately because the displacement direction of the piezoelectric vibrator elements, the excitation electric field and the mechanical terminal conditions are not taken into account.
In addition, there are problems with the equipment constants K.sub.R and K.sub.L in that they must be computed in advance on the basis of measurements of the impedance characteristic and their values vary from viscoelastic medium to viscoelastic medium.